From gym + muscles.
gym muscles (plurale tantum)
- (slang, derogatory) Large, aesthetic, well-developed muscles, especially ones that are believed to be overly large or impractical.
2003, Elaine Viets, Shop till you Drop: a Dead-end Job Mystery, page 84:He was six feet tall and strong, but without the gnarled gym muscles Helen hated.
2006, Alex Duval, Vampire Beach: Initiation, page 94:Sienna reached out, grabbed Jason's hand, and ran his fingers across her taut abs. Jason tried to look unimpressed. "Gym muscles," Tyler scoffed.
2008, Elaine Viets, Murder With Reservations, page 144:Physical labor gave her muscles, but not sculpted gym muscles.
2009, Grant McCrea, Drawing Dead, page 67:Sure, the sick Italian fuck had muscles on top of the muscles all over his chunky irritating self. But they were gym muscles. Bench-press-with- your-iPod muscles.
2009, Mehmet Murat Somer, The Kiss Murder, page 138:He didn't have puffed-up gym muscles. His strapping body had a rippling stomach, ridged like a tray of baklava.
2010, Alexander Irvine, Iron Man 2: The Junior Novel, page 109:[…] and he realized that, unlike a lot of big guys with gym muscles, the guard could take a punch.